Creative Evolution
The Mechanism of Creative Evolution. By C. C. Hurst. (Cambridge University Press. 21s.)
TUE idea of organic evolution was not new when Darwin published the Origin of Species ; but it was merely an idea. Darwin gave it substance and showed, by the theory of natural selection, not only how it could, but must have come about. That theory, however, one of the greatest conceptions in the history of human thought, was by itself insufficient—as Darwin himself, in his superb honesty, realized—because Nature, if she is to select a species, must have a choice from which to select, and Darwin could find no plausible theory of the origin of variations. He was far from satisfied with his own more pedestrian version of Lamarck's mystical "inherit- ance of acquired characters." Further, as even he failed to realize, the current view, which he tacitly accepted, of the blending in the offspring of the two parents' characters must have inevitably involved a constant reduction of the amount of variation.
For nearly forty years, consequently, Darwinism was left in the air, since none with critical ability could doubt the accu- mulating evidence of organic evolution nor the irresistible common sense of natural selection, and yet none could find a wholly acceptable theory of the origin of variations. With the rediscovery of Mendel's work, its early enthusiasts thought they had found the causes of evolution in the segregation, independent assortment, and (wholly hypothetical) "un- packing," of the genes (hereditary factors). Almost imme- diately afterwards De Vries, Gates, and thereafter a host of others discovered " mutations " and proceeded to develop theories of evolution by sudden " spontaneous "jumps.
There were thus four protagonists in the field—the surviving Lamarckians, still stoutly maintaining that if the fathers ate sour grapes the children's teeth were set on edge ; the Dar- winians, hesitantly accepting that view in a modified form, but unable to reject natural selection ; the Mendelians, and the mutationists. All schools, however, accepted the fact of organic evolution. With the accumulation of evidence and precision of thought, Lamarckism died a natural death, and the factor of natural selection, though the degree of its scope was much debated, came to be generally acknowledged. Meanwhile Mendelians and mutationists blended into one camp. There was a general, vague—very vague—agreement that mutations were the origin of variation, that Mendelism juggled with them, and that natural selection created species out of the reniainder. Precision and substantial reconcile- ment came with the publication, a few years ago, of Fisher's Genetical Theory of .Natural Selection, which converted the vague agreement into a solid mass of unassailable data and theory, and the modifications and refinements of Haldane and Sewall Wright—all three mathematical attacks. There were also the supplementary biological suggestions of Elton, who demonstrated first that the populations of species fluctuate periodically and greatly, and showed how this would increase the survival chances of mutations ; and next suggested that sometimes natural selection is reversed—that animals select their environment, instead of it selecting them.
In the book under review we have a further supplement to the general concept of evolution, filling in yet another area of unmapped territory. Dr. Hurst, who is a geneticist of dis- tinction and long standing, here elaborates in detail his own and others' work on various species and genera of plants which have thus evolved by polyploidy "—by the reduplica- tion, that is, of whole sets of chromosomes, the physical vehicles of the hereditary genes. It is the first time (the reviewer thinks), that this particular method of evolution— and so great a process must have not one, but many causes— has been so fully and coherently covered, and it will be especially welcome to general biologists who have failed to keep pace with this important study. It is to be doubted, however, whether polyploidy or any form of transmutation (a change involving whole chromosomes) can have played very much part in the evolution of animals. Gene mutations (ultra-microscopic changes, within the chromosomes, of a bio-chemical or bio-physical nature) seem more likely, and, moreover, the evidence indicates that they, in spite of their minuteness, produce the greater changes as well as the smaller variations. There is, for instance, less difference (compared with the parent stock) in those types of rose or evening primrose which have doubled or quadrupled their chromosomes sets than there is in the blind or wingless mutants of the fruit-fly Drosophila, whose chromosomes exhibit no abnormality to the microscope.
The general reader will undoubtedly find much of interest in even this rather technical side-line, while the main theme of the book will fascinate him, since it lives up to its title and concentrates with lucidity on the physical mechanism of heredity and its mutations—besides giving a constructive resume, as readable as the rest, of what is known or guessed of the origin of life, and some delightful speculations on the future evolution of mind.
The studies on the inheritance of musical ability are valuable and interesting ; but judgement must be reserved on the Leicestershire study of the genetics of general intelligence. The chromosomes of the horse are given as 56 on p. 22, and as 60 on p. 50. The latter, according to the latest counts, is
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